PURPOSE: To obtain a new PNB esterase obtainable from a recombinant host cell, etc., in a purified form having extremely high specific activity from an extract of a bacterium of the genus Bacillus.

CONSTITUTION: This PNB esterase in a substantially purified form is prepared from a recombinant host cell capable of expressing the PNB esterase. The esterase is expressed by ATCC1105/PNB106R transformed with plasmid PNB106R given by the example and is purified by ammonium sulfate fractionation, pH treatment, anion-exchange chromatography, gel filtration and adsorption-desorption chromatography. The enzyme has a monomer having a molecular weight of about 54,000 daltons, substrate specificity to loracarbef PNB ester and cefaclor PNB ester, is strongly inhibited by phenylmethylsulfonyl fluoride, diethyl pyrocarbonate, etc., and is stable at about 4-50°C and at pH about 5.5 to 8.5.